Honeycomb extrusion dies are commonly used to form cellular or honeycomb ceramic substrates for use in catalytic converters, which are utilized in exhaust systems of internal combustion engines. In order to reduce back pressure within the exhaust system, it is necessary that the cell walls or webs of the substrate have a substantially thin cross-sectional dimension so as to provide a substantially large open frontal area. However, the thin walled structure must be protected so as to withstand normal automotive impact requirements. One way to protect the inner cell walls is to provide an outer skin of increased thickness, thus providing additional strength to withstand external loads.
The present invention relates to honeycomb die designs, and more particularly to die designs offering improved control over the delivery of plasticized powdered ceramic batch materials toward the skin-forming regions of such dies, in order to better control the thicknesses and flow properties of the co-extruded skin layers needed to effectively surround and protect the honeycomb core portions of the extruded honeycomb structures.
Current methods of providing a sufficient flow of skin-forming ceramic material on extruded ceramic honeycombs have involved modifications of the pin or outlet side of the extrusion die. Generally, such methods comprise the cutting away of material from the pin side of the die and/or attaching throttling plates to the inlet side of the die bodies to provide a thick, smooth skin area on the extruded part. U.S. Pat. Nos. 5,219,509 to Cocchetto et al and 6,455,124 B1 to Beall et al are representative of such methods. Also U.S. Pat. No. 4,915,612 to Gangeme et al utilizes a mask associated with the discharge face of the die and a forming plate positioned on the inlet face of the feed holes to produce an outer skin on a honeycomb substrate.
However, many of these know methods are subject to various handling and extrusion problems, including damaging the die pins during the cutting away process which renders the die completely useless. Partial pins, formed by a square cell pattern when being cut by a rounded skin pattern, are subject to excessive wearing. Also, the cutting of the pin surface results in a variable surface finish on the completed die, which produces changing extrudate flow rates as the die is slowly eroded by the flow of abrasive batch through the die. Also of concern are variations in flow from one die to the next, which can cause high variability in selects between dies and therefore reduced material utilization in the forming plants. Further, where such variations in flow exist, the ceramic skins are not always well adhered to the honeycomb core portions of the extruded parts.
It is known to increase the flow of skin-forming material by widening the discharge slots in the outer skin-forming region to improve the wet strength of the formed part and to provide improved adherence of the skin on the ceramic honeycomb part. Such widening techniques, however, are expensive in terms of tooling and machine time. Further, the necessary machining is irreversible. That is, the die cannot be re-converted to a uniform slot width design, which may be necessary due to surface finish wear that eventually results in excessive flow of batch material in the skin-forming region.